This invention relates to a film useful as a heat sealing material, a heat-sealing laminate using the film as a substrate thereof and a process for heat sealing the film and the laminate to a plastic container.
Generally, in plastic containers which are widely used for seal packing foodstuffs such as dairly products and confections, there have chiefly been used covers made of aluminum foil. When plastic containers are sealed with covers of aluminum foil, the covers are generally bonded to the containers with a hot-melt type adhesive or solvent type adhesive. In the container using such an adhesive when the cover is removed from the container, the adhesive remains on the container. The adhesive thus left on the container is not desirable hygienically in the least and is possible to do harm to the human body. On the other hand, a process for heat sealing such plastic containers with a plastic film has also been widely known. In this case, the problem ascribable to use of an adhesive is not entailed. But, since the sealing is made between two similar materials, the seal thus formed certainly has ample strength and, for this reason, suffers from a disadvantage that desired peeling of the cover from the container becomes difficult to the extent of making the container opening quite inconvenient. For this process, therefore, the requirements that the peeling of the cover from the container should be easy (easy seal peeling) and that the cover provides high heat sealing property must be satisfied simultaneously.
To satisfy these contradictory requirements, proper selection of a film as the sealing material and proper selection of conditions for heat sealing are necessary.
The expression "easy seal peeling" as used in this invention means the property of a sealed container that in a normal state, the container remains in a tightly sealed state and, when the cover is peeled from the container, the cover can easily be removed by hand without any breakage to the cover or the container proper, and the expression "normal state" means the state in which the sealed container is transported or otherwise handled in the distribution channel.